1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, an LCD device for supplying light to a liquid crystal panel by using a plurality of light emitting diodes (LEDs).
2. Description of the Related Art
In general, an LCD is a display device for displaying a desired image by adjusting a light transmittance of pixels by separately supplying data signals according to image information to the pixels arranged in a matrix form.
Accordingly, the LCD device includes a driving unit for driving the liquid crystal display panel with the pixels arranged in the matrix form thereon and the pixels.
The liquid crystal panel includes a thin film transistor (TFT) array substrate and a color filter substrate attached in a facing manner with a uniform cell gap maintained therebetween, and a liquid crystal layer formed in the cell gap between the array substrate and the color filter substrate.
In this case, a common electrode and pixel electrodes are formed on the liquid crystal display panel, which is formed as the array substrate and the color filter substrate are attached, to apply electric field to the liquid crystal layer.
Thus, in a state that voltage is applied to the common electrode, when the voltage of the data signal applied to the pixel electrode, liquid crystal of the liquid crystal layer is rotated due to dielectric anisotropy according to the electric field between the common electrode and the pixel electrodes, thus transmitting light or interrupting light by pixels to allow for display of characters or images.
In this case, the LCD device is a light receiving device that displays an image by adjusting the transmittance of light coming from an external light source, rather than emitting light by itself, so it requires a device, namely, a backlight, for supplying light to the liquid crystal panel.
The backlight is divided into a side type backlight in which lamps are disposed at one side or at both sides of the liquid crystal panel and light is reflected, diffused and concentrated through a light guide plate, a reflection plate and optical sheets so as to be transmitted to a front surface of the liquid crystal panel and a direct type backlight in which lamps are disposed on a rear surface of the liquid crystal panel so as to be directly transmitted to the front side of the liquid crystal panel.
FIG. 1 is a perspective view of a general side type backlight.
As shown in FIG. 1, the side type backlight includes a light guide plate 41 disposed on a rear surface of the liquid crystal panel (not shown), lamps 25 disposed at the side of the light guide plate 41, a reflection plate 42 disposed on a rear surface of the light guide plate 41, a lamp holder (not shown) for fixing the lamps 25 to the side of the light guide plate 41, and wirings 27 applying power to the lamps 25.
Light generated from the lamps 25 are made incident to the side of the light guide plate 41 made of a transparent material, and the reflection plate 42 disposed on the rear surface of the light guide plate 42 reflects light transmitted to the rear surface of the light guide plate 41 to an upper surface of the light guide plate 41, thus reducing a loss of light and improving uniformity.
In general, as the lamps 25 applied to the side type backlight or the direct type backlight, a tube type cold cathode fluorescence lamp (CCFL) having a length corresponding to a longer side distance or a shorter side distance of the liquid crystal panel is applied, and the CCFL generates white light by power supplied through the wirings 27 at both sides thereof.
In this case, when the CCFL is applied as a light source of the backlight, a fluorescent discharge tube encapsulating mercury (Hg) gas with argon (Ar), neon (Ne), or the like, added thereto is used as a fluorescent tube in order to use a penning effect. In this case, electrodes are formed at both ends of the fluorescence discharge tube, and a negative electrode is formed to be large in a plate form, and when voltage is applied thereto, charge particles within the discharge tube collides with the negative electrode in the plate shape, like a sputtering phenomenon, to generate secondary electrons, and the generated secondary electrons excite the nearby elements to form plasma. These elements emit strong ultraviolet rays, and as the emitted ultraviolet rays excite phosphor again, the phosphor emits visible rays.
However, the backlight using the CCFL has not good color reproduction because the light source, namely, the CCLF, does not have good light emission characteristics. In addition, the size and capacity of the fluorescent lamp are limited, a backlight of a high luminance cannot be obtained.
In addition, because mercury applied as phosphor to the CCFL is harmful to a human body, it cannot cope with the environment regulation which is gradually strengthened.
Recently, a light emitting diode (LED) receives much attention as a light source of the backlight. The LED has a longer life span than that of the CCFL, and because it operates at a DC of 5V, it does not need an inverter.
Namely, the high luminance LED has a longer life span than that of the CCFL and consumes power 20% of the existing product, and because the high luminance LED does not need additional equipment such as an inverter or the like, it is advantageous for making a product thinner and effectively use an inner area. Also, its color implementation capability is evaluated to be superior to the CCFL, and the full-fledged regulation since 2006 worldwide supports the adoption of the LED backlight.
FIG. 2 is a sectional view schematically showing a general LCD using an LED backlight.
As shown in FIG. 2, in the general side-type LED backlight LCD, an LED array 20 generating light, an LED printed circuit board (PCB) 21 for driving the LED and a reflective plate 42 are installed on a lower cover 50, and an LED housing reflector 55 is attached to an upper portion of the lower cover 50 at an upper side of the LED array 20.
A light guide plate 41 is installed in a direction in which the LED array 20 outputs light, and in this case, a plurality of LEDs constituting the LED array 20 are positioned between LEDs at a certain interval in order to obtain an overall luminance uniformity of the backlight.
The reflective plate 42 positioned under the light guide plate 41 serves to reflect light outputted from the LED array 20 toward the lower cover 50 to transmit it toward optical sheets 43. The LED housing reflector 55 reflects light outputted toward the upper portion of the lower cover 50 from the LED array 20 to transmit it toward the optical sheets 43.
A liquid crystal panel 10 composed of a TFT array substrate and a color filter substrate is mounted on an upper portion of the LED backlight thusly configured and coupled with an adhesive tape 11 through an upper case (not shown) to form the LCD.
Meanwhile, with the development of the display industry, demand for a thinner display and reduction in a production cost is increasing.
In order to make the existing LED backlight thinner, the thickness of respective main components must be minimized. However, the performances such as the minimization of thickness, an optical efficiency and mechanic reliability are traded off each other, causing a limitation in minimizing the thickness.
In addition, in order to reduce the cost of the existing backlight, a processing time must be reduced, a production yield must be improved, and the cost of row materials must be cut down. However, there is a limitation in the shortening the processing time and improving the production yield, and the pressure of an increase in the cost of the raw materials is also increasing, making it difficult to cut down the costs.
Thus, with these problems, the related art fails to cope with the current demand for thinner display and cost cutdown.
Namely, in the general LCD illustrated in FIG. 2, if a thickness d1 of the LCD is reduced to meet the demand for making the display thinner, an assembly process, in particularly, an assembly processibility of the LED housing reflector 55, would deteriorate. Also, if the size of the LED array 20 is reduced, the quantity of light would be reduced, and also, because the LED PCB 21 and the reflective plate 42 overlaps, reflection efficiency is degraded.